It is well known that some current modems operating in accordance with G.992.1 and G.992.2 standards implement a transmitter with an Inverse Discrete Fourier Transform (IDFT) size greater than the one specified by the standards. These modems may implement the transmitter in either the upstream or downstream direction, or both. There are many reasons that may influence this decision. For example, for an upstream channel the standard IDFT size is 64 points. However, an IDFT size greater than 64 points may be justified for two reasons. The first reason is hardware symmetry with the downstream channel, since the downstream channel requires a larger Discrete Fourier Transform (DFT). The second reason is ease of implementation of different Annexes of the G.992.1 and G.992.2 standards with the same data path.
It is well know that Asymmetric Digital Subscriber Loop (ADSL) systems suffer from some performance degradation whenever there is a mismatch between the transmitter IDFT and receiver DFT sizes. This is particular true on short loops where the channel is Inter-Symbol Interference (ISI)—Inter-Channel Interference (ICI) dominated, as the DFT/IDFT size mismatch increases the ISI-ICI effects. Laboratory tests show that if the transmitter uses an IDFT other than that suggested by the standard on short loops, data rate penalties of approximately 25 to 30% are experienced.
However, if the transmitter IDFT size is known a priori, that is before training the receiver equalizer, the receiver DFT could possibly be changed to match the size of the transmit IFFT. This allows the performance to be optimized regardless of the IDFT size used by the transmitter. However, this solution has two potential problems. The first problem is the IDFT size may simply not be known a priori. The problem particularly applies to G.992.1 and G.992.2 modems, while new G.dmt-bis and G.lite-bis standards have G.hs codepoints defined for exchanging information about the transmitter IDFT. The second problem is the receiver may not be able to change the DFT size on the fly due to “Millions of Instructions per Second” (MIPS) or hardware limitations.
Thus there is a need for a system and method for allowing the transmitter to use an IDFT having a size different to that specified in the standard, while reducing data rate penalties. It is an object of the present invention to obviate or mitigate at least some of the above-mentioned disadvantages.